Skybus is certainly the cheapest way. A car is only convenient if you have someone to drive you back and forth. If not, forget it. I don't recommend taxi since it can be hard to spot in a crowd, though you might not have to manage your luggage alone then.
The Skybus is generally the cheapest way to get there unless you live in the Essendon area. Make sure you allocate plenty of time though, because the duration of the trip from Southern Cross is very dependent on traffic and how many other people want to go to the airport!
Yes please bring on the train line. It is 3/4 of the way there, but when will it EVER get ALL the way there? We fly up & down to Melbourne, visiting our 2 daughters & 2 grand-daughters 5/6 times a year. That makes the bus expensive, esp' when these no Seniors discount!